(A) Field of the Invention
The present invention relates to a method for designing gratings, and more particularly, to a method for designing gratings by using an optimization algorithm to select structural parameters to design gratings.
(B) Description of the Related Art
The capabilities of process control play an important role in product yield. According to the International Technology Roadmap for Semiconductor (ITRS) report, the precision requirement for overlay measurement is 3.5 nanometers for a node of 130 nanometers, and 3.2 nanometers for a node of 90 nanometers. As the line width becomes smaller and smaller, the precision requirement for the overlay measurement is 2.3 nanometers for a node of 65 nanometers in the next-generation process. It is very difficult for the conventional bright-field metrology systems to meet the precision requirement due to limitations in image resolution. Scatterometry has good repeatability and reproducibility and is proposed for the overlay measurement on the next-generation process. Consequently, increasing importance is placed on improving the overlay measurement precision based on scatterometry to meet increasingly rigorous precision requirements.
The prior art uses numerical algorithms such as principle component analysis, neural network analysis or interpolation analysis to acquire valuable information from the finite data to increase the comparison accuracy. These analyses considered the measured data to be signal without noise, but the measured data in fact includes irrelevant noise, which influences the data-acquiring accuracy during the subsequent data processing. In particular, the actual fabrication process generally includes process variation, for example, the refractive index of fabrication material deeply depends on the variation of the fabrication condition, and a little variation in the fabrication condition can cause a large variation in the intensity of the diffraction spectrum.